The Common Mezzanine Card (CMC) standard (as described in IEEE standard P1386, as amended from to time) describes, among other things, the mechanics or form factor of a mezzanine card family that can be used interchangeably on a variety of host computer boards. These mezzanine cards can be used to provide modular I/O and/or general function expansion.
The mezzanine card's local bus can be based on the Peripheral Component Interconnect (PCI) specification or other specifications as developed in the future. PCI is a high speed local bus used by a variety of microprocessors. The PCI specification calls for a board that plugs into a carrier in a perpendicular fashion. Perpendicular boards are not usable for low profile computing applications, so the CMC standard defines a slim, modular, parallel mezzanine card, where I/O can be done via the front bezel and/or through connectors to the host computer.
For the purpose of this specification and for maximum clarity of terminology throughout this specification, the CMC card will be called the “CMC board” and the present invention's “daughter card” will be called as such.
The CMC board envelope space defines a space that may be occupied by the PCB, associated electronic components, and required cooling gap. The total CMC board envelope space is divided into two parts, the component envelope and the I/O envelope. The component envelope's maximum height is 8.2 mm. The I/O envelope's maximum height is 13.5 mm. The maximum depth of the I/O envelope is 31.0 mm. Within such standardized envelope dimension, the designer must work within good design constraints (such as stacking heights) for realistic performance, i.e. the real usable space is less than the standard envelope dimensions.
The component envelope is where electronic components for board core functions, are normally placed. No component is allowed to protrude through this envelope, except the CMC connectors, standoffs and bezel retention screws.
The I/O envelope is normally used for mounting I/O connectors on the CMC bezel. No component is allowed to protrude outside the I/O envelope (with the exception of the CMC bezel and I/O connectors mounted on the bezel, which protrude beyond the face of the bezel).
Conventionally, mezzanine cards based on the CMC standard have had a fixed core function and a fixed I/O capability. Examples of board core functions include: digital up-converters, digital down-converters, analog/digital converters, digital/analog converters, echo cancellers. Examples of I/O capabilities include: LVDS (Low Voltage Differential Signal), ECL (emitter-coupled logic) to TTL (transistor-transistor logic), differential ECL to TTL, SCSI, fibre optic's FPDP. In some applications, it is desirable to use a CMC board for its core functionality but require a solution that incorporates different I/O capabilities. One solution has been to produce the same basic CMC board but with different I/O functionality on the front panel. Another solution has been to route the I/O through the connectors to the host computer and use a separate standalone I/O adapter.